In a TDMA cellular radiotelephone system, each radio channel is divided into a series of time slots, each of which contains a burst of information from a data source, e.g., a digitally encoded portion of a voice conversation. The time slots are grouped into successive TDMA frames having a predetermined duration. The number of time slots in each TDMA frame is related to the number of different users that can simultaneously share the radio channel. If each slot in a TDMA frame is assigned to a different user, the duration of a TDMA frame is the minimum amount of time between successive time slots assigned to the same user. The successive time slots assigned to the same user, which are usually not consecutive time slots on the radio carrier, constitute the user's digital traffic channel (or DTC), which may be considered a logical channel assigned to the user. As described in more detail below, digital control channels (DCCs) can also be provided for communicating control signals, and such a DCC is a logical channel formed by a succession of usually non-consecutive time slots on the radio carrier.
In North America, these features are currently provided by a digital cellular radio telephone system called the digital advanced mobile phone service (D-AMPS), some of the characteristics of which are specified in the interim standard IS-54B, “Dual-Mode Mobile Station-Base Station Compatibility Standard”, published by the Electronic Industries Association and Telecommunications Industry Association (EIA/TIA). Interim Standard (IS) 136 (promulgated by the Telecommunications Industry Association) adds a Digital Control Channel (DCCH) to IS-54B. References to IS-54B in this document are meant to incorporate IS-136.
According to IS-54B, each TDMA frame consists of six consecutive time slots and has a duration of 40 milliseconds (ms). Thus, each radio channel can carry from three to six DTCs (e.g., three to six telephone conversations) depending on the source rates of the speech coder/decoders (codecs) used to digitally encode the conversations. Such speech codecs can operate at either full-rate or half-rate, with full-rate codecs being expected to be used until half-rate codecs that produce acceptable speech quality are developed. The TDMA cellular system operates in a buffer-and-burst, or discontinuous-transmission, mode: each mobile station transmits (and receives) only during its assigned time slots. At full rate, for example, a mobile station might transmit during slot 1, receive during slot 2, idle during slot 3, transmit during slot 4, receive during slot 5, and idle during slot 6, and then repeat the cycle during succeeding TDMA frames. Therefore, the mobile station, which may be battery-powered, can be switched off, or sleep, to save power during the time slots when it is neither transmitting nor receiving. In the IS-54B system in which the mobile does not transmit and receive simultaneously, a mobile can sleep for periods of at most about 27 ms (four slots) for a half-rate DTC and about 7 ms (one slot) for a full-rate DTC.
Note that the term mobile station is used herein to refer to a radio unit in a communications system, which includes radio units that are “stationary” or “fixed”, and is not limited to being “mobile.” The term “mobile” unit is used herein merely because of its wide acceptance and clear meaning in the communications arts.
In addition to voice or traffic channels, cellular radiocommunication systems also provide paging access, or control, channels for carrying call-setup messages between base stations and mobile stations.
In general, the transmission rate of the DCC need not coincide with the half-rate and full-rate specified in IS-54B, and the length of the DCC slots may not be uniform and may not coincide with the length of the DTC slots. The DCC may be defined on an IS-54B radio channel and may consist, for example, of every n-th slot in the stream of consecutive TDMA slots. In this case, the length of each DCC slot may or may not be equal to 6.67 ms, which is the length of a DTC slot according to IS-54B. Alternatively (and without limitation on other possible alternatives), these DCC slots may be defined in other ways known to one skilled in the art.
FIG. 1 shows a general example of a forward DCC configured as a succession of time slots 1, 2, . . . N, . . . belonging to a particular DCC. These DCC slots may be defined on a radio channel such as that specified by IS-54B, and may consist, for example, of every n-th slot in a series of N consecutive slots. The DCC slots shown in FIG. 1 are organized into superframes (SF)′, and each superframe includes a number of logical channels that carry different kinds of information. One or more DCC slots may be allocated to each logical channel in the superframe.
Of the slots in a Superframe available for signaling, some are designated by the base station (BMI) for broadcast (point-to-multipoint) messaging and the rest for point-to-point messaging. In order for the base station to be able to notify (or “page”) a mobile station (MS) of an incoming call (or other impending transaction), the mobile station is assigned to one and only one of the slots in a Superframe available for point-to-point messaging on the forward DCC (i.e., that portion of the DCC used to transmit messages from the base station to the mobile station).
As used herein, the terms “page” and “page message” refer to one or more slots of data transmitted from a base station over the point-to-point messaging channel that contains information intended to signal one (or possibly more) of a plurality of mobile stations that such mobile stations have an incoming call. (An incoming call can be, for example, a voice call or any other type of incoming call capable of being serviced by the base station and mobile stations.) The term “non-page message” refers to any slots of data transmitted over the point-to-point messaging channel that are not “page messages”.
FIG. 1 also shows an exemplary downlink superframe, which includes at least three logical channels: a broadcast control channel (BCCH) including six successive slots for overhead messages; a paging channel (PCH) including one slot for paging messages; and an access response channel (ARCH) including one slot for channel assignment and other messages. The remaining time slots in the exemplary superframe of FIG. 1 may be dedicated to other logical channels, such as additional paging channels or other channels. Since the number of mobile stations is usually much greater than the number of slots in the superframe, each paging slot is used for paging several mobile stations that share some unique characteristic, for example, the last digit of the MIN.
Although IS-54B provides for digital traffic channels, more flexibility is desirable in using digital control channels having expanded functionality to optimize system capacity and to support hierarchical cell structures, i.e., structures of macrocells, microcells, picocells, etc. The term “macrocell” generally refers to a cell having a size comparable to the sizes of cells in a conventional cellular telephone system (e.g., a radius of at least about 1 kilometer), and the terms “microcell” and “picocell” generally refer to progressively smaller cells. For example, a microcell might cover a public indoor or outdoor area, such as a convention center or a busy street, and a picocell might cover an office corridor or a floor of a high-rise building.
From a radio coverage perspective, macrocells, microcells, and picocells may be distinct from one another or may overlap one another to handle different traffic patterns or radio environments. Each of these types of cells has a base station which transmits at least one control channel. Thus, a number of neighboring control channels are present for a mobile or remote unit to evaluate as a possible replacement for the current serving control channel to which it is locked.
Accordingly, DCCs will be periodically evaluated by the mobile station for possible control channel reselection. For example, when in an idle state i.e., switched on but not making or receiving a call, a mobile station in an IS-54B system tunes to and then regularly monitors the strongest control channel (generally, the control channel of the cell in which the mobile station is located at that moment) and may receive or initiate a call through the corresponding base station. Re-tuning is required when the mobile station receives signals from the serving base station (registered station) at such a degraded level that the mobile station is forced to tune to a control channel of a nearby alternative base station.
The initial tuning and subsequent re-tuning to control channels are both accomplished automatically by scanning all the available control channels at their known frequencies to find the best control channel. The terms scan or scanning as used in this document, can refer to, for example, signal strength measurement, actual signal decoding, or any other method of evaluating a signal.
When a control channel with good reception quality is found, the mobile station remains tuned to this channel until the quality deteriorates again. In this way, mobile stations are relatively continuously in communication with the cellular system.
According to a more recent innovation in cell reselection disclosed in U.S. Pat. No. 5,353,332 to Raith and Muller, each control channel in each cell is configured to broadcast information, including system parameters, about the presence, if any, of other cells and the characteristics of those cells including minimum quality criteria, power requirements, etc. Typically, information about the presence of other cells is broadcast about neighboring cells. For instance, a neighboring cell may be adjacent to, overlapping, or non-contiguous from the broadcasting cell.
A mobile periodically scans, during idle mode, the neighboring control channels in the coverage area that the mobile is located in to determine in which cell it should be locked (or registered). This process is known as background scanning. Each control channel includes a neighbor list. The neighbor list identifies other control channels which mobiles locked to that control channel should periodically evaluate. Thus, a mobile may continuously select cells to be locked to based on the existing location of the mobile and quality criteria, such as received signal strength, associated with the cells. The cell to which the mobile may be locked is the cell in which the mobile satisfies the quality criteria associated with the cell.
While in the idle state, and in addition to evaluating control channels as potential reselection candidates, a mobile station must monitor the control channel for paging messages addressed to it. When the base station needs to “page” the mobile station, that is, notify the mobile station that it has an incoming call (or other impending transaction), the base station transmits a “page message” or a “hard page” in the assigned slot for the mobile station. Under quiescent conditions, the mobile station need only monitor this assigned slot in the Superframe. Thus, the mobile station is able to “sleep” while the other 31 slots of the Superframe are being transmitted. In addition, because every other Superframe transmitted (i.e., every primary Superframe) by the base station is followed by a Superframe (secondary Superframe) having identical point-to-point paging slots, the mobile unit can sleep during every other entire Superframe. For other types of point-to-point traffic, however, the mobile station is not assigned to a specific slot in a Superframe; rather, when it is expecting a “non-page message,” i.e., not a message intended to notify the mobile station of an incoming call (or other impending transaction), from the base station, the mobile station is required to search for non-page messages addressed to itself in each slot available for point-to-point messaging within the Superframe.
Currently, mobile stations are hashed to a slot in the Superframe (called a PCH Subchannel) where the mobile station expects to receive page traffic. Nominally, the mobile station is required to read this same slot in every other Superframe.
FIG. 2 depicts a block diagram of the various elements in a time division multiple access digital control channel. A TDMA Frame 200 is depicted which is made up of six slots 202, 204, 206, 208, 210, 212. Each slot 202, 204, 206, 208, 210, 212 is transmitted through the communications channel, consisting primarily of air, during a 6.67 ms period of time, such that the TDMA Frame 200 is 40 ms in duration. In accordance with current TDMA conventions, a TDMA channel is made up of every third slot within the TDMA Frame. Thus, slots 1 and 4 (202, 208) are a part of one TDMA channel, slots 2 and 5 (204, 210) part of another TDMA channel, and slots 3 and 6 (206, 212) yet another.
Within each TDMA channel, groups of 32 TDMA blocks (and thus 32 slots) comprise a Superframe 214, having a duration of 640 ms. A total of three Superframes, one per TDMA channel, is transmitted every 640 ms. Within each Superframe 214, a portion of the slots is designated as the Broadcast Channel (BCCH), another portion Reserved, and another portion the point-to-point messaging channel. Each mobile unit monitoring a particular base station is assigned to monitor a particular PCH subchannel, i.e., a particular slot within the point-to-point messaging channel.
As an example, slot 24 (216) may be the monitored PCH subchannel for a particular group of mobile units within a cell (assuming for a given case slot 24 is part of the point-to-point messaging channel). The PCH subchannel may contain any of a plurality of point-to-point communications encoded in 324 bits, which make up the PCH subchannel.
The mobile unit, is not always monitoring the digital control channel (DCCH) for Page Messages. Each slot in a given digital control channel has a specified function (in accordance with IS-136). Some, called broadcast control channel (BCCH) slots, are allocated to carry overhead information to all mobile stations that may be monitoring a particular digital control channel (point-to-many). Others, called Short Message Service, Paging, and Access Response Channel (SPACH) slots, are used by the base station to carry point-to-point Messages to a specific mobile station. Together the BCCH and the SPACH (as possibly some Reserved slots) comprise a Superframe. Each Superframe is made up of a total of thirty-two slots (in a full-rate DCCH), each allocated as a BCCH slot or a SPACH slot (or possibly a Reserved slot). Superframes are transmitted in pairs referred to as Primary and Secondary Superframes, respectively. In a quiescent system, the mobile station is only required to monitor a single SPACH slot (called its Paging Channel (PCH) Subchannel) in every other Superframe, i.e., in every Primary Superframe.
In digital cellular systems, there are several standards requiring the mobile station to search for a system while registered on another (background scanning). This search takes place on the mobile station without any assistance from the base station (as in reselection or handoff). Furthermore, this search takes place without any coordination between the mobile station and the base station.
There are currently several specifications requiring this type of background scanning. For example, Intelligent Roaring for ANSI 136 utilizes a database to determine the priority of a system. While registered on certain low priority systems, the mobile station must search for a system of higher priority.
Another example of a specification requiring background scanning is the Inter-Network Roaming Selection specification from GSM North America (GSM NA). This specification requires a priority database to determine the best system in AMPS and GSM 1900. While registered to a low priority system, the mobile station must search another protocol for a higher priority system. The ANSI-136 Intelligent Roaming and Inter-Network Roaming Selection specifications are the first of many specifications dealing with prioritized system selection requiring background scanning.
The drawback of background scanning, however, is that, during the period of time the mobile station is retrieving identification parameters from the base system under investigation (or evaluation) as a possibly better system, the mobile station can miss pages on the system to which it is registered. This aspect of the background scanning is addressed in the Intelligent Roaming specification by forcing the base station to repeat a missed page after five seconds. This gives a mobile station in the TDMA/AMPS environment a second chance to receive the page.
The GSM NA specification does not address this possibility of missed pages. Therefore, a mobile station conforming to the GS NA standard can fatally miss a page if the unrepeated page goes undetected by the mobile station.